1.10

• Diffusion

  The movement of molecules from an area of high concentration to an area of low concentration
• Osmosis

  The movement of water molecules from an area of high water concentration to an area of low water concentration through a semi-permeable membrane
• Active transport

  The movement of molecules against a concentration gradient, using energy
• For many single-celled organisms, diffusion, osmosis, and active transport are all that is needed to exchange materials with their environment because they have a relatively large surface area compared to the volume of the cell
• This allows sufficient transport of molecules into and out of the cell to meet the needs of the organism
• Surface area to volume ratio

  The ratio of the surface area to the volume of an object
• The surface area to volume ratio is very important in biology
• It makes a big difference to the way animals can exchange substances with the environment
• Surface area to volume ratio is also important when you consider how energy is transferred by living organisms, and how water evaporates from the surfaces of plants and animals
• The ratio of surface area to volume falls as objects get bigger

  In a small object, the surface area to volume (SA:V) ratio is relatively large
• This means that the diffusion distances are short and that simple diffusion is sufficient for the exchange of materials
• As organisms get bigger

  The surface area to volume ratio falls
• As the distances between the centre of the organism and the surface get bigger, simple diffusion is no longer enough to exchange materials between the cells and the environment
• Effective exchange surfaces

  • Have a large surface area over which exchange can take place
  • Have a thin membrane or being thin to provide a short diffusion path
  • In animals, have an efficient blood supply that moves the diffusing substances away from the exchange surfaces and maintains a steep concentration (diffusion) gradient
  • In animals, are ventilated to make gas exchange more efficient by maintaining steep concentration gradients
• Adaptations for exchanging materials

  • Fitzroy river turtle's rear opening with finger-like folds and rich blood supply for gas exchange
  • Human lungs with alveoli having large surface area and rich blood supply for gas exchange
  • Small intestine villi providing large surface area, short diffusion paths and rich blood supply for material exchange
  • Fish gills with stacks of thin filaments and rich blood supply for gas exchange
  • Plant roots with large surface area and root hair cells for efficient water and mineral uptake
  • Plant leaves with flat, thin structure, air spaces, and stomata for effective gas and solute exchange